The invention is based on a metal halide lamp according to the preamble of claim 1. It relates in particular to metal halide lamps with a discharge vessel made from quartz glass or ceramic, which is often accommodated in an outer bulb.
1. Prior Art
DE-A 43 27 534 has already disclosed a metal halide lamp which uses as its fill, for photo-optical purposes, AlI3 and/or AlBr3 together with metal halides of thallium, cesium and/or rare-earth metals for high color temperatures of over 5000 K.
In metal halide lamps which are used as UV emitters, iron is frequently used as the most important UV-radiation source. In this connection, it is known, for example from EP-B-543,169, to add other UV-emitters, such as manganese, bismuth, thallium or tin, in order to avoid the blackening which is formed with iron.
To achieve warm white and neutral white luminous colors with color temperatures of below 5000 K, metal halide discharge lamps often contain sodium. For example, U.S. Pat. No. 3,575,630 describes a fill containing halides of the metals Na, Tl and Zr. Metal halide discharge lamps with a discharge vessel made from glass and a sodium-containing fill have the drawback that sodium diffuses through the discharge vessel, with the result that the service life of the lamps is reduced. The diffusion of sodium has to be reduced by additional measures, for example by shielding the supply conductor in the vicinity of the discharge vessel. This increases the production costs of the lamp. A further drawback of sodium-containing metal halide discharge lamps is their relatively poor color rendering. Typical values are Ra=70 for the general color rendering index and R9=0 for the specific red color rendering index.
2. Description of the Invention
The object of the present invention is to provide a metal halide lamp according to the preamble of claim 1, which contains very little or no sodium and in particular yet nevertheless provides a color temperature of below 5000 K (corresponding to a warm white or neutral white luminous color).
This object is achieved by means of the characterizing features of claim 1. Particularly advantageous configurations are given in the dependent claims.
According to the invention, the metal halide fill contains as the principal or only constituent manganese in the form of Mn halide. As an alternative to the known utilization of the intensive spectral lines in the UV range, the spectral lines of manganese in the visible spectral range are for the first time used to improve the general color rendering index Ra. As a result of (possibly substantially) dispensing with the use of sodium, it is possible to do without the additional measures for reducing the diffusion of sodium. The improved red rendering (R9) is attributable primarily to the fact that there is a series of Mn lines in the wavelength range of greater than 603 nm.
A particular advantage is that the UV radiation of manganese can additionally be used to increase the temperature of the discharge vessel. This is achieved by producing an envelope (which is often an additional outer bulb and/or the discharge vessel itself) from UV-impermeable material, for example from hard glass or doped quartz glass. The UV radiation is thus absorbed in the envelope and is to a large extent returned to the discharge vessel. Consequently, the temperature of the cold spot is increased, which is of benefit to the light efficiency. With manganese as the only metal halide, it is typically possible to achieve a very high color temperature of more than 8000 K at a high Ra of more than 90. In total, it is possible to achieve an Ra greater than 95 and an R9 greater than 90.
Advantageously, manganese is combined with further halides of the elements Cs, Dy, Tl, Ho, Tm and, if appropriate, small quantities of sodium. In this case, the molar ratio Mn/Na should be  greater than 1, preferably  greater than 2. In this case, Mn is used to completely or partially substitute Na, because significant spectral lines of Mn lie in the visible spectral range, very close to the sodium-D lines. These fills containing a plurality of components are eminently suitable for use in general illumination to produce warm white or neutral white luminous colors with a color temperature of between approximately 3000 and 4500 K. Manganese forms in this case an essential component of the metal halide fill; in particular its proportion amounts to at least 20% by weight of the entire metal halide fill.
Preferably, the amount of Mn in the fill is from 0.01 to 50 xcexcmol per cm3 of the volume of the discharge vessel.
In a particularly preferred embodiment, up to 30 xcexcmol per cm3 of Cs are added to the fill. As an alternative, or in addition, one or more of the following components (usually in halide form) are added to the fill: up to 35 xcexcmol per cm3 of Dy, and/or up to 15 xcexcmol per cm3 of Tl, and/or up to 18 xcexcmol per cm3 of Ho, and/or up to 18 xcexcmol per cm3 of Tm. It is thus possible to fine-tune the desired Ra and R9.
The halogens used to form halides are preferably iodine and/or bromine.
Advantageously, the volume between discharge vessel and outer bulb is evacuated. It is thus possible to achieve a particularly high color rendering index Ra. As an alternative, the volume between the discharge vessel and outer bulb may contain a gas fill, in particular inert gas, which results in an increase in the service life. In a particularly preferred embodiment, the gas fill comprises from 10 to 90 kPa N2 (cold) or from 5 to 70 kPa CO2 (cold).